Extreme adaptations for probable visual courtship behaviour in a Cretaceous dancing damselfly

Courtship behaviours, frequent among modern insects, have left extremely rare fossil traces. None are known previously for fossil odonatans. Fossil traces of such behaviours are better known among the vertebrates, e.g. the hypertelic antlers of the Pleistocene giant deer Megaloceros giganteus. Here we describe spectacular extremely expanded, pod-like tibiae in males of a platycnemidid damselfly from mid-Cretaceous Burmese amber. Such structures in modern damselflies, help to fend off other suitors as well as attract mating females, increasing the chances of successful mating. Modern Platycnemidinae and Chlorocyphidae convergently acquired similar but less developed structures. The new findings provide suggestive evidence of damselfly courtship behaviour as far back as the mid-Cretaceous. These data show an unexpected morphological disparity in dancing damselfly leg structure, and shed new light on mechanisms of sexual selection involving intra- and intersex reproductive competition during the Cretaceous.

originating below Pt; MA long, ending on posterior wing margin below base of RP2; MP short, one or two cells long; CuA reduced to oblique vein; Pt very small, less than half length of surrounding cells; all tibiae spectacularly expanded, covered with two brown bands, in pod-like sclerite except on metatibiae where of semi-circular shape.
Description. Specimen NIGP164757 (Figs 1A and 2A-F), body well preserved. Head dark ( Fig. 2A), 2.74 mm long and 0.97 mm wide; eyes 0.89 mm wide, well separated by gap of 0.79 mm; ocelli located low between eyes; antenna three segmented, with segment 1 short and stout, segment 2 stout and 0.75 mm long, segment 3 slim and 0.76 mm long. Legs well developed, profemur 4.11 mm long and armed with long spines in basal part, protibia 2.66 mm long and 0.61 mm wide (Fig. 2D), tarsus 0.62 mm long (claws excluded); mesofemur 5.03 mm long, mesotibia 3.06 mm long and maximum 0.81 mm wide, tarsus 0.74 mm long (Fig. 2E); metafemur 9.02 mm long, metatibia 6.63 mm long and maximum 2.96 mm wide, tarsus 0.91 mm long (Fig. 2F); paired long spines present on tibia and tarsi; tibia armed with about ten pairs of spines; tarsi slightly curved, three segmented, with length of third tarsomere equal to first two tarsomeres combined; basal tarsomere armed with two pairs of long spines, second and third tarsomere armed with four pairs of spines; apical claws symmetrical, 0.12-0. 16 (Fig. 2H), Ax0 close to wing base, Ax1 1.29 mm distal of Ax0, Ax2 0.64 mm distal of Ax1; no secondary antenodal and antesubnodal crossveins present. Five postnodal and five postsubnodal crossveins present before Pt, somewhat aligned. One postnodal and one postsubnodal crossvein present distal of Pt, non-aligned (Fig. 2I). Arc angular and aligned with Ax2. DC basally closed, free and rectangular, 0.8 mm long and 0.18 mm wide. Subdiscoidal cell free and elongate, 0.81 mm long and 0.2 mm wide. Nodal structures well preserved, Sn aligned with Cr. Midfork (base of RP3/4) slightly basal of N; RP3/4 curved, reaching posterior wing margin just below base of Pt brace. Base of IR2 aligned with Sn, one cell and 0.93 mm distal of midfork; IR2 basally straight but distally zigzagged, ending on posterior wing margin just below Pt. RP2 originating three cells distal of Sn, equidistant between N and Pt, lying 3.6 mm distal of Sn. IR1 originating below end of Pt and six cells distal of base of RP2. RP1 with strong angle below pterostigmal brace. MA basally slightly curved but distally zigzagged, reaching posterior wing margin just below base of RP2. MP curved and short, covering two cells. CuA short, reduced to oblique vein. Pt quite small, rectangular and hyaline, 0.47 mm long and 0.22 mm wide.
Specimen SMNS Bu-137 ( Fig. 3) shares all wing characters of specimen BA16200 (Fig. 4) besides following differences: wing short and 11.5 mm long; MP one or two cells long; IR1 originated below Pt base, and five cells distal base of RP2.
Remarks. Y. huangi has a short vein IR1 originating below the distal side of the pterostigma which is only present in a few damselflies, viz. the platycnemidid Palaeodisparoneura burmanica Poinar, Bechly and Buckley, 2010, the hemiphlebiid Burmahemiphlebia zhangi Zheng et al. 27 , the dysagrionid Burmadysagrion zhangi Zheng, Wang and Nel, 2016, and the recent perilestid genus Perilestes Hagen in Selys-Longchamps, 1862. Perilestes species have the base of RP3/4 and IR2 distal of the nodus, differentiating them from Y. huangi 29 . Burmadysagrion zhangi has a unique discoidal cell (anterior and posterior sides not parallel, and the basal side longer than the distal side), long MP and CuA, star-shaped Pt, and can thus be easily distinguished from Y. huangi 30 . Y. huangi has a rectangular discoidal cell, short CuA, RP1 with strong angle below Pt base as in both P. burmanica and Burmahemiphlebia zhangi 23,30 , but differs from the latter two in having only one postnodal crossvein distal of Pt, a smaller Pt, and expanded male tibiae. However, Burmahemiphlebia zhangi has a basally open discoidal cell in the forewing, closed in the hindwing, quite different from Y. huangi, while P. burmanica has a closed discoidal cell more like Y. huangi.
Very few modern male damselflies have expanded tibia, namely Platycypha Fraser, 1949 (Chlorocyphidae), Platycnemis Burmeister, 1839, Proplatycnemis Kennedy, 1920, Copera Kirby, 1890, Matticnemis Dijkstra, 2013 (tibiae weakly expanded) and Pseudocopera Fraser, 1922 (Platycnemidae: Platycnemidinae) 32 . Affinities of Y. huangi with Chlorocyphidae are excluded because of the different wing venation (numerous antenodals, long CuA, no angular RP1 in Chlorocyphidae) 33 . The wing venation of Y. huangi is very similar to that of the fossil platycnemid P. burmanica. Platycnemididae, called white-legged damselflies, currently consist of over 400 species, widely distributed in the Old World [34][35][36] . They are currently divided into six modern subfamilies 35 plus the Cretaceous Palaeodisparoneurinae Poinar et al. 9 . Despite the fact that the type specimen of P. burmanica is a male without expanded tibiae, Y. huangi has all the characters listed in the diagnosis of this subfamily 23 . On the other hand, Y. huangi strongly differs from modern Platycnemidinae in the very short veins IR1, MP and CuA (apomorphies of Palaeodisparoneurinae), and the broad area between RP1 and IR2. Here we establish a new genus for Y. huangi and place it in the Palaeodisparoneurinae. This attribution implies a convergent evolution in the expanded tibiae between Y. huangi and modern Platycnemidinae.

Discussion
Expanded legs are not common in male insects and are normally used for courtship. Some extant male nomiine bees have expanded tibiae using for clasping the females in case of separation during courtship 37,38 . Within recent damselflies, male Platycypha and Platycnemis species have expanded tibiae used for courtship displays 39 . Platycypha has all six tibiae expanded, but not as much as Y. huangi. Male P. lacustris Förster, 1914 has the most expanded tibia within Platycypha species, with the outer dilation of hind tibiae 3-5 times wider than the shaft 40,41 . This small size does not approach Y. huangi. Also, Platycypha normally has a different coloration of the inner and outer sides of the tibiae for different functions 42 , unlike Y. huangi which has hyaline tibiae and similar pigmented colour on both sides. Male Platycnemis are characterized by feather-like tibiae and wide mid and hind tibiae. Besides the Japanese endemic P. echigoana Asahina, 1955, all Platycnemis species have more or less broadened, flattened and symmetrical tibiae in the middle and hind legs 43,44 . Platycnemis phasmovolans Hämäläinen 2003 has the most expanded tibiae among modern Platycnemididae, 5.5 mm long and 2 mm broad at the widest point in the hindleg 44 . However, this tibial size is smaller than that of Y. huangi, which is 6.6 mm long and 3 mm wide. Unlike Platycypha and Platycnemis, the tibiae of Y. huangi are hyaline, partly covered with two narrow brown bands, and asymmetric with a pod-like shape, especially the hind tibiae which are semi-circular in shape. However, male Platycnemis have more expanded tibiae, suggesting closer similarities in the sexual behaviour with Y. huangi.
During courtship, male Platycypha caligata Selys-Longchamps 1853 waves the white anterior surface of all six laterally enlarged tibiae at the females, but uses the posterior surface of the tibiae for intra-sexual signaling during territorial defence 42,[45][46][47][48] . Similarly, male East Asian Platycnemis species with expanded, feather-like tibiae 49 well differentiated from the females, exhibit a strong sexual dimorphism 50 . The males display their white legs in a fluttering flight in front of females before mating 39,44,50 . By morphological inference, the six extremely expanded tibiae of Y. huangi could also have a signaling function for courtship displays. Platycypha has all six tibiae expanded, but all less so than Y. huangi in size. Platycnemis has more expanded mid and hind tibiae, but is still smaller than Y. huangi. These more expanded fossil tibiae suggest an extreme adaptation for courtship behaviour. More importantly, unlike Platycypha and Platycnemis, the tibiae of Y. huangi are asymmetric and pod-shaped, especially the hindleg tibia with a semi-circular outline. This pod-like shape would make waving slower due to air resistance. Y. huangi waving its giant pod-like tibiae would make males more easily noticed and attract female attention (Fig. 5), increasing mating opportunities and implying sexual selection.
The tibial shape of Y. huangi also resembles the wings of some members of the extinct neuropteran families Kalligrammatidae and Saucrosmylidae (lacewings). The hindleg tibia of Y. huangi have a semi-circular shape, almost the same as the hindwings of the Jurassic saucrosmylid Daohugosmylus castus Liu et al. 51 . The fore and Furthermore, the tibiae of Y. huangi are hyaline and partly covered with two narrow brown bands, making them even more like pigmented wings. In addition, there is an eye-shaped spot in the middle of the hindleg, quite like the wing spots in Kalligrammatidae and some recent butterfly eyespots. These well-developed eyespots were and are used to make a conspicuous and contrasting display to intimidate vertebrate predators or protect the body by deflecting an attack to the wings [54][55][56] . Deflective eyespots in butterflies and fossil lacewings are smaller than deimatic ones and both are never on the legs, but dragonflies are predators with good eyesight, and the tiny ones in Y. huangi may have less to do with paralleling fossil lacewings in deflecting nearby predators and more to do with raising the interest of females (cf. peacock eyespots 57 ). Some recent damselflies, such as male Calopteryx hamorrhoidalis, with higher wing pigmentation, are more likely to defend their territories and obtain more matings 58 . That none of the pigmented tibiae in Y. huangi are damaged, however, suggests they did not precipitate an aggressive response.
Our new fossil indicates that the Platycnemis-type of courtship behaviour originated at least 100 million years ago. The exaggerated tibiae probably also made them fly slowly. They probably found it less easy to escape from new predators (small birds more efficient than pterosaurs), thus adding more risk in their fancy flight.

Methods
Photographs were taken using a Zeiss Stereo Discovery V16 microscope system with Zen software. In most instances, incident and transmitted light were used simultaneously. All images are digitally stacked photomicrographic composites of approximately 40 individual focal planes obtained using the free software Combine ZP for a better illustration of the 3D structures. The line drawings were prepared from photographs using image-editing software (CorelDraw X7 and Adobe Photoshop CS6). Specimen NIGP164757 is housed at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS). Specimen BA16200 is currently in NIGAPS but will be finally deposited in the Lingpoge Amber Museum in Shanghai (China). Specimen SMNS Bu-137 is housed at State Museum of Natural History in Stuttgart (Germany).
The nomenclature of the dragonfly wing venation used in this paper is based on the interpretations of Riek 59 and Riek & Kukalová-Peck 60 , as modified by Nel et al. 61 and Bechly 62 . The higher classification of fossil and extant Odonatoptera, as well as family and generic characters followed in the present work, are based on the phylogenetic system proposed by Bechly 62 and Dijkstra et al. 63 for the phylogeny of extant Zygoptera.